Genome engineering requires the ability to insert, delete, substitute and otherwise manipulate specific genetic sequences within a genome, and has numerous therapeutic and biotechnological applications. The development of effective means for genome modification remains a major goal in gene therapy, agrotechnology, and synthetic biology (Porteus et al. (2005), Nat. Biotechnol. 23: 967-73; Tzfira et al. (2005), Trends Biotechnol. 23: 567-9; McDaniel et al. (2005), Curr. Opin. Biotechnol. 16: 476-83). A common method for inserting or modifying a DNA sequence involves introducing a transgenic DNA sequence flanked by sequences homologous to the genomic target and selecting or screening for a successful homologous recombination event. Recombination with the transgenic DNA occurs rarely but can be stimulated by a double-stranded break in the genomic DNA at the target site. Numerous methods have been employed to create DNA double-stranded breaks, including irradiation and chemical treatments. Although these methods efficiently stimulate recombination, the double-stranded breaks are randomly dispersed in the genome, which can be highly mutagenic and toxic. At present, the inability to target gene modifications to unique sites within a chromosomal background is a major impediment to successful genome engineering.
One approach to achieving this goal is stimulating homologous recombination at a double-stranded break in a target locus using a nuclease with specificity for a sequence that is sufficiently large to be present at only a single site within the genome (see, e.g., Porteus et al. (2005), Nat. Biotechnol. 23: 967-73). The effectiveness of this strategy has been demonstrated in a variety of organisms using chimeric fusions between an engineered zinc finger DNA-binding domain and the non-specific nuclease domain of the FokI restriction enzyme (Porteus (2006), Mol. Ther. 13: 438-46; Wright et al. (2005), Plant J. 44: 693-705; Urnov et al. (2005), Nature 435: 646-51). Although these artificial zinc finger nucleases stimulate site-specific recombination, they retain residual non-specific cleavage activity resulting from under-regulation of the nuclease domain and frequently cleave at unintended sites (Smith et al. (2000), Nucleic Acids Res. 28: 3361-9). Such unintended cleavage can cause mutations and toxicity in the treated organism (Porteus et al. (2005), Nat. Biotechnol. 23: 967-73).
A group of naturally-occurring nucleases which recognize 15-40 base-pair cleavage sites commonly found in the genomes of plants and fungi may provide a less toxic genome engineering alternative. Such “meganucleases” or “homing endonucleases” are frequently associated with parasitic DNA elements, such as group I self-splicing introns and inteins. They naturally promote homologous recombination or gene insertion at specific locations in the host genome by producing a double-stranded break in the chromosome, which recruits the cellular DNA-repair machinery (Stoddard (2006), Q. Rev. Biophys. 38: 49-95). Meganucleases are commonly grouped into four families: the LAGLIDADG (SEQ ID NO: 55) family, the GIY-YIG family, the His-Cys box family and the HNH family. These families are characterized by structural motifs, which affect catalytic activity and recognition sequence. For instance, members of the LAGLIDADG (SEQ ID NO: 55) family are characterized by having either one or two copies of the conserved LAGLIDADG (SEQ ID NO: 55) motif (see Chevalier et al. (2001), Nucleic Acids Res. 29(18): 3757-3774). The LAGLIDADG (SEQ ID NO: 55) meganucleases with a single copy of the LAGLIDADG (SEQ ID NO: 55) motif (“mono-LAGLIDADG (SEQ ID NO: 55) meganucleases”) form homodimers, whereas members with two copies of the LAGLIDADG (SEQ ID NO: 55) motif (“di-LAGLIDADG (SEQ ID NO: 55) meganucleases”) are found as monomers. Mono-LAGLIDADG (SEQ ID NO: 55) meganucleases such as I-CreI, I-CeuI, and I-MsoI recognize and cleave DNA sites that are palindromic or pseudo-palindromic, while di-LAGLIDADG (SEQ ID NO: 55) meganucleases such as I-SceI, I-AniI, and I-DmoI generally recognize DNA sites that are non-palindromic (Stoddard (2006), Q. Rev. Biophys. 38: 49-95).
Natural meganucleases from the LAGLIDADG (SEQ ID NO: 55) family have been used to effectively promote site-specific genome modification in plants, yeast, Drosophila, mammalian cells and mice, but this approach has been limited to the modification of either homologous genes that conserve the meganuclease recognition sequence (Monnat et al. (1999), Biochem. Biophys. Res. Commun. 255: 88-93) or to pre-engineered genomes into which a recognition sequence has been introduced (Rouet et al. (1994), Mol. Cell. Biol. 14: 8096-106; Chilton et al. (2003), Plant Physiol. 133: 956-65; Puchta et al. (1996), Proc. Natl. Acad. Sci. USA 93: 5055-60; Rong et al. (2002), Genes Dev. 16: 1568-81; Gouble et al. (2006), J. Gene Med. 8(5):616-622).
Systematic implementation of nuclease-stimulated gene modification requires the use of genetically engineered enzymes with customized specificities to target DNA breaks to existing sites in a genome and, therefore, there has been great interest in adapting meganucleases to promote gene modifications at medically or biotechnologically relevant sites (Porteus et al. (2005), Nat. Biotechnol. 23: 967-73; Sussman et al. (2004), J. Mol. Biol. 342: 31-41; Epinat et al. (2003), Nucleic Acids Res. 31: 2952-62).
I-CreI is a member of the LAGLIDADG (SEQ ID NO: 55) family which recognizes and cuts a 22 base-pair recognition sequence in the chloroplast chromosome, and which presents an attractive target for meganuclease redesign. The wild-type enzyme is a homodimer in which each monomer makes direct contacts with 9 base pairs in the full-length recognition sequence. Genetic selection techniques have been used to modify the wild-type I-CreI cleavage site preference (Sussman et al. (2004), J. Mol. Biol. 342: 31-41; Chames et al. (2005), Nucleic Acids Res. 33: e178; Seligman et al. (2002), Nucleic Acids Res. 30: 3870-9, Arnould et al. (2006), J. Mol. Biol. 355: 443-58, Rosen et al. (2006), Nucleic Acids Res. 34: 4791-4800, Arnould et al. (2007). J. Mol. Biol. 371: 49-65, WO 2008/010009, WO 2007/093918, WO 2007/093836, WO 2006/097784, WO 2008/059317, WO 2008/059382, WO 2008/102198, WO 2007/060495, WO 2007/049156, WO 2006/097853, WO 2004/067736). More recently, a method of rationally-designing mono-LAGLIDADG (SEQ ID NO: 55) meganucleases was described which is capable of comprehensively redesigning I-CreI and other such meganucleases to target widely-divergent DNA sites, including sites in mammalian, yeast, plant, bacterial, and viral genomes (WO 2007/047859).
A major limitation of using mono-LAGLIDADG (SEQ ID NO: 55) meganucleases such as I-CreI for most genetic engineering applications is the fact that these enzymes naturally target palindromic DNA recognition sites. Such lengthy (10-40 bp) palindromic DNA sites are rare in nature and are unlikely to occur by chance in a DNA site of interest. In order to target a non-palindromic DNA site with a mono-LAGLIDADG (SEQ ID NO: 55) meganuclease, one can produce a pair of monomers which recognize the two different half-sites and which heterodimerize to form a meganuclease that cleaves the desired non-palindromic site. Heterodimerization can be achieved either by co-expressing a pair of meganuclease monomers in a host cell or by mixing a pair of purified homodimeric meganucleases in vitro and allowing the subunits to re-associate into heterodimers (Smith et al. (2006), Nuc. Acids Res. 34:149-157; Chames et al. (2005), Nucleic Acids Res. 33:178-186; WO 2007/047859, WO 2006/097854, WO 2007/057781, WO 2007/049095, WO 2007/034262). Both approaches suffer from two primary limitations: (1) they require the expression of two meganuclease genes to produce the desired heterodimeric species (which complicates gene delivery and in vivo applications) and (2) the result is a mixture of approximately 25% the first homodimer, 50% the heterodimer, and 25% the second homodimer, whereas only the heterodimer is desired. This latter limitation can be overcome to a large extent by genetically engineering the dimerization interfaces of the two meganucleases to promote heterodimerization over homodimerization as described in WO 2007/047859, WO 2008/093249, WO 2008/093152, and Fajardo-Sanchez et al. (2008). Nucleic Acids Res. 36:2163-2173. Even so, two meganuclease genes must be expressed and homodimerization is not entirely prevented.
An alternative approach to the formation of meganucleases with non-palindromic recognition sites derived from one or more mono-LAGLIDADG (SEQ ID NO: 55) meganucleases is the production of a single polypeptide which comprises a fusion of the LAGLIDADG (SEQ ID NO: 55) subunits derived from two meganucleases. Two general methods can be applied to produce such a meganuclease.
In the first method, one of the two LAGLIDADG (SEQ ID NO: 55) subunits of a di-LAGLIDADG (SEQ ID NO: 55) meganuclease can be replaced by a LAGLIDADG (SEQ ID NO: 55) subunit from a mono-LAGLIDADG (SEQ ID NO: 55) meganuclease. This approach was demonstrated by replacing the C-terminal subunit of the di-LAGLIDADG (SEQ ID NO: 55) I-DmoI meganuclease with an I-CreI subunit (Epinat et al. (2003), Nucleic Acids Res. 31: 2952-62; Chevalier et al. (2002), Mol. Cell 10:895-905; WO 2003/078619). The result was a hybrid I-DmoI/I-CreI meganuclease which recognized and cleaved a hybrid DNA site.
In the second method, a pair of mono-LAGLIDADG (SEQ ID NO: 55) subunits can be joined by a peptide linker to create a “single-chain heterodimer meganuclease.” One attempt to produce such a single-chain derivative of I-CreI has been reported (Epinat et al. (2003), Nucleic Acids Res. 31: 2952-62; WO 2003/078619). However, as discussed herein as well as in Fajardo-Sanchez et al. (2008), Nucleic Acids Res. 36:2163-2173, there is now evidence suggesting that this method did not produce a single-chain heterodimer meganuclease in which the covalently joined I-CreI subunits functioned together to recognize and cleave a non-palindromic recognition site.
Therefore, there remains a need in the art for methods for the production of single-chain heterodimer meganucleases derived from mono-LAGLIDADG (SEQ ID NO: 55) enzymes such as I-CreI to recognize and cut non-palindromic DNA sites.